I model in Sculptris, which I find particularly intuitive. While I model, I look at pictures of actual slugs and am reminded of anatomical details not present in my concepts. I add those, and I will later realise I want the eyes to be bigger.

I imported my model to meshmixer to scale it and give it a perfectly flat base.

Then I import to Cura to slice the model for 3D printing, with support structures.